General Mills is taking a stand against a proposed state constitutional amendment to ban same-sex marriage, becoming the most prominent corporate voice making such a public declaration. Chief executive Ken Powell voiced the company’s opposition Wednesday at a General Mills function attended by 400 gay and lesbian professionals, followed Thursday by a Web letter from the company’s vice president for global diversity and inclusion, Ken Charles. “We do not believe the proposed constitutional amendment is in the best interests of our employees or our state economy,” Charles wrote. “We value diversity. We value inclusion.”